In a conventional lead-acid battery, a battery case defines interior cells, and spaced plates or electrodes are located within each cell submerged in a battery electrolyte. The lead-acid battery generally uses highly reactive sponge lead for the negative electrode, lead dioxide for the positive electrode, and a liquid solution of sulfuric acid for the electrolyte. The generalized equation for the electrochemical reaction within the cell when discharging and when being charged is: ##STR1##
This electrochemical reaction causes a measurable change in the specific gravity or density of the electrolyte. In determining the state-of-charge or operational readiness of a lead-acid battery, information regarding the specific gravity or density of the electrolyte in addition to voltage current, and temperature thus is desired. The specific gravity of the electrolyte in a lead-acid battery changes between a low value of 1.05-1.15 when the battery is in a discharged state and a high value of 1.25-1.35 when the battery is in a fully charged state.
Knowing this, it has been commonplace to measure with a hydrometer the specific gravity of the battery electrolyte to determine the vitality of the battery. To use the hydrometor, the battery filler cap must be removed in order to insert a hydrometer tube into the battery cell so as to draw the electrolyte into the hydrometer. The hydrometer has a float that bobs within the electrolyte to a specific depth, and this float is visually sighted against a scale to determine a specific gravity reading. For good accuracy, the reading must be adjusted for the difference between the electrolyte temperature and a normalized temperature, commonly considered to be 25.degree. C., since the specific gravity varies substantially as a function of temperature.
Use of the hydrometer can be difficult and/or dangerous and/or tedious. For example, the battery can be located in a vehicle chassis where it could be difficult to obtain the electrolyte sample and/or have good sight of the positioned hydrometer. The battery electrolyte must be taken from the battery so that the possibility exists that it can be spilled onto the vehicle or onto the person, causing damage to either, or it can be contaminated before being put back into the battery. Moreover, as the float must be free to bob or move within the hydrometer, the hydrometer must be precisely oriented in a vertical manner when taking the reading. Once the electrolyte is discharged from the hydrometer back into the battery cell, the reading is lost and there is no permanent record. The required removal of the battery filler caps, etc. can be quite tedious if a large number of batteries must be charged and checked on a regular basis. Moreover, a true test would have to be made on all six battery cells; but this is not done as a matter of course and only a rough approximation of the battery condition is obtained by testing just one of the battery cells. Moreover, a maintenance-free battery does not even have a removable cap so that the float type hydrometer technique of testing the battery cannot even be used.